Plasma chromatographs are electrical devices such as ion mobility spectrometers or detectors capable of detecting trace levels of vapor or gas in gaseous mixtures. The configuration of such a detector includes a cell consisting of ionization, reaction, and drift regions along with a shutter grid and a Faraday ion collector or cup and associated processing electronics. The trace vapors or gases are ionized in the reaction region to produce product ions which are sorted according to mobility in the drift region. The mobility of a given ion is inversely proportional to the time it takes to traverse the drift region under the influence of an electrostatic field. Present ion mobility detectors are capable of detecting either positive or negative ions but not both at the same time. Generally, the operation of an ion mobility detector is similar to the operation of a time of flight mass spectrometer, the obvious difference being that a time of flight mass spectrometer operates under vacuum conditions where the mean free path of the contained gases is many times the dimensions of the gas container, while the ion mobility detector operates generally at atmospheric pressure where the mean free path of the contained gases is a small fraction of the dimensions of the container. More particularly, a carrier gas, normally purified atmospheric air, is introduced into the ion mobility detector with a gaseous sample of a material whose identify is to be characterized by the ion mobility detector. This gaseous mixture is introduced into the aformentioned reaction region so that it flows past and is exposed to an ionization source. As a result, portions of both the carrier gas and the sample are directly ionized by the ionization source. However, as known to those practicing in this art, the characteristics of the carrier gas and the sample are usually such that the molecules of the carrier gas are more easily directly ionized by the ionization source than are molecules of the sample. Initially the ions are contained within the reaction region because the shutter grid is electrically charged to repel the ions. Since the mean free path of the gas molecules within the reaction region is many times smaller than the dimensions of the region there are multiple collisions between the molecules of the carrier and sample gases. As also known to those skilled in the art, the tendency of these collisions is to transfer the ion charge from the carrier molecules to the sample molecules, thereby ionizing the sample gas mainly by this secondary ionization process.
The charged particles or ions, now mainly derivatives of the sample, are accelerated to a terminal velocity under the influence of a field potential gradient within the reaction region towards an ion injection grid shutter which separates the reaction region from the drift region. As previously mentioned, the shutter grid is normally electrically charged to repel ions and thus prevent the transfer of ions from the reaction region to the drift region. Periodically, the shutter grid is energized for a short time period to permit a pulse of ions to pass therethrough into the drift region. Here, the ions, under the influence of an electrostatic drift field, are accelerated to an electrometer detector which terminates the drift region. The time of arrival of each ion at the electrometer detector, relative to the time the shutter grid was opened, is determined by the ion's mobility through the nonionized gas occupying the drift region. The heavier ions move more slowly through the drift region and arrive at the electrometer detector after longer drift times than lighter ions. It is thus possible to characterize the ions and hence the sample by observing the time between the opening of the shutter grid and the arrival of ions at the electrometer detector.
The ion mobility detector is normally in the form of a hollow cylindrical structure which defines the various regions. The electrical field gradient within the regions is such as to cause either positive or negative ions to drift towards the electrometer detector. For example, if negative ions are to be detected then a field gradient of a first polarity is used. If positive ions are to be detected then a field gradient of opposite polarity is used.
Problems associated with using ion mobility detectors for environmental sampling are sensitivity and specificity to the sample compound of interest. In particular, more than one compound is often of interest during the detection process. Many times, certain of these compounds are more sensitive to the positive ion spectrum, others are more sensitive in the negative ion spectrum, and still others have characteristic signatures in both spectra. Under these conditions it is desirable that both positive and negative ions be detected from a common sample.